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EIA persistent equine disease in the West

The mention of equine infectious anemia (EIA) or swamp fever holds a special kind of fear for horse owners.

With no vaccine and no cure for the disease, a positive diagnosis of EIA is essentially a death sentence as an affected horse must be either euthanized or quarantined for life in fly-proof facilities to avoid infecting other horses.

Some of the increased detection came from chuckwagon associations making testing mandatory for competitions in 2012. Other cases were found when the Canadian Food Inspection Agency (CFIA) tested herds that had come into contact with positive horses.

But what is it about this particular disease that makes screening so important?

“The EIA virus is a blood-borne virus similar to HIV (human immunodeficiency virus),” explains Dr. Katharina Lohmann, a large animal internal medicine specialist and associate professor in the Western College of Veterinary Medicine (WCVM). “The EIA virus is a retrovirus, which means it incorporates its own DNA into host cells and uses them to manufacture more virus. It’s also known as a lenti virus, which means it is a slow-moving disease.”

EIA is a disease of interest to human HIV research because a horse’s immune system can adapt to and challenge the EIA virus even though it can’t clear it completely. This results in a cyclical pattern of acute sickness followed by a relatively healthy phase in infected horses.

“During the acute phase of EIA, horses will appear lethargic, feverish, and have decreased appetites. Blood work will show anemia and a decreased platelet count,” says Lohmann.

Contrary to popular belief, many horses that contract EIA do get quite sick. While it’s true that some horses may have mild symptoms that are easily overlooked, a number of horses died or were humanely euthanized during Saskatchewan’s most recent outbreak due to the effects of the disease itself.

As the disease progresses, appetite suppression and cyclical fever can lead to chronic weight loss. There are also damaging side effects from the horse’s own immune system as it fights the virus.

“Producing a vaccine is tough because the virus mutates frequently within the body,” adds Lohmann. “We can easily make a vaccine for specific strains of the virus but not for the mutations.”

Although EIA is spread primarily through insects, mosquitoes do not play much part in the spread of the disease.

“In relative terms,” Lohmann says, “EIA takes a ‘significant’ amount of blood to transmit. Biting flies such as horseflies and deerflies do most of the transmission – their mouthparts are large and they tend to chew at the skin, so their bite creates a pool of blood. It’s also painful, which means a horse may swat the fly off before it’s done feeding. If the fly moves on to the next horse, it may spread the disease.”

Insects themselves do not act as a host for the EIA virus. Instead, there must be a mechanical transmission of blood – either by natural vectors such as insects, or through human error such as sharing of needles, improperly sterilized dental equipment or stomach tubes between horses.

Given its method of transmission, it’s logical that recent EIA outbreaks have centred on forest-fringe areas — the northern parkland areas of Saskatchewan and Alberta, the Peace River region of Alberta and B.C. and the B.C. interior — all places in Canada where horseflies and deerflies are most active.

There are always questions about how and when routine EIA screening should be conducted, and what effects it has on the equine industry in general. In certain parts of the U.S., specifically southern Gulf regions, a negative Coggins test is routinely required for moving horses for any reason — breeding, showing, trail riding or change of ownership.

“A test always assesses a horse’s current state,” says Lohmann. “Because this is a slow-moving disease, a recent Coggins can be a valuable indicator.”

Theoretically, EIA should be an easy disease to eradicate if all horses in Canada were tested and all positive animals were culled. However, as Lohmann points out, there are isolated, semi-wild herds that may act as pools or reservoirs for the disease.

“Basically, the horses that are routinely tested are those that are least likely to need it.”

This reality is both good news and bad news for horse owners. For the most part, routine EIA screening will ensure their herds’ safety. But as seen in the recent outbreak, low numbers of the disease can promote a false sense of security among horse owners. Until there’s more widespread testing of horses in all facets of the horse industry, it’s up to individual owners to keep their herds safe.